The great thing about those urn things you see on the counters of small cafes is that you always have a supply of boiling water, so you can make a refreshing cup of tea at any time.

Tea, as we know, sustains the British nation, or at least those of her citizens doing anything useful. So wouldn't it be great if we could have a giant national urn somewhere and distribute boiling water to every building in the land?

This would be problematic. For reasons of hygiene and robustness, the piping would probably have to be made from copper, and that's expensive. And, in any case, China is taking most of it these days, so we probably wouldn't have enough of it anyway.

And then you would have to keep the water boiling as it made its way across vast tracts of wilderness, such as Oxfordshire, on its way to civilisation in Hammersmith. So we'd probably need regional urn substations. I'm afraid it just wouldn't work... or would it? Let's say we build our giant urn in a place called Chodford. We can use the steam from the boiling water to drive a turbine, which can turn a generator to make electricity. This is easy to distribute, with wires and pylons, and it can be made readily available at the wall socket.

So I plug my kettle into this, and it makes... boiling water. There is boiling water at each end of the system, but none in between, only electricity. This is why electricity is brilliant. But it does only one thing: it is energy's envoy, continuing its good work in other far-flung places.

Actually, we can simplify this further. Let's say the big urn is heated by coal. So a man lights a fire under a kettle in Chodford, but the cup of Rosie is made in Hammersmith. My kettle is electric, obviously, but the water is actually heated by the coal.

Sorry if this all seems a bit ruddy obvious, but I'm trying to explain how the Fisker Karma works without being interrupted by Jeremy. This is the only place where that's possible.

So the Karma is an electric car, but it also has a petrol engine. This is already confusing, so think of it like this. The engine is the Chodford power station, and the electric motors driving the wheels are my kettle. The wheels are ultimately driven by the petrol that goes in the tank, because that's where the energy comes from. But it gets to the wheels through the good offices of electricity.

So a man lights a fire under a kettle in Chodford, but the cup of Rosie is made in Hammersmith.

Yes, there is a battery, and you can recharge that from the mains if you want, in which case the wheels are perhaps turned by coal or gas or nuclear fusion, or even a windmill or a waterfall. Electricity is just doing its regular day-to-day job as the envoy.

If I had a Karma, I doubt I'd ever plug it in. I'd just use the engine/generator. But the battery is still useful, because it's like the spring in a clockwork toy car. It stores energy that might otherwise be wasted - during braking, say - for use later, in electrical form. But remember that you can only brake if the car is moving, and it only moves in the first place because of the energy in the petrol.

And now for Sir Isaac Newton. Any pub physicist will tell you that 'twas he who said that energy can't be created or destroyed. This is true in an experiment in the lab, but, in the real world, energy can be thrown away to the universe, as heat.

The petrol engine in a car works by converting the heat of combustion to mechanical work, but it can never convert all of it. A lot of it is thrown away. What we save we usually pass to the wheels by gearboxes, clutches and shafts, which are necessary to manage the many shortcomings of the engine's performance characteristics - it has to idle, it only works over a limited range, and so on.

In the Fisker, the work of all these mechanical bits is done by the generator, the battery and the motors. They are a different means of turning petrol into motion. That's all there is to it. Forget the fatuous distinctions between electricity, fossil fuel and so-called alternative energy. It's just energy.

And, in the Fisker, more of the petrol's energy is rescued, and it's managed more effectively than is the case in other cars. That's why it can accelerate like a Maserati Quattroporte, even though it only has a 2.0-litre engine.